High efficiency traveling wave tubes



Feb. 6, 1962 A. EICHENBAUM 3,020,439

HIGH EFFICIENCY TRAVELING WAVE TUBES Filed July 50, 1958 5 Sheets-Sheet l /VPI/T a 4 2z 2a 22 a2 jlxll ill IIII /iam/ 42ml III lll 300m( Z7 a f g. INVENTOR.

ARIE- L. EIEHEMBAUM Feb. 6, 1962 A. l.. EICHENBAUM 3,020,439

l HIGH EFFICIENCY TRAVELING WAVE TUBES Filed July 5o, 195e 5 sheets-sheet 2 ,f 55 400144200 20 u "-M-lllllllll |I||||||| IFILM- M INVENTR ARIE L. EIEH ENBAUM Feb. 6, 1962 A. EICHENBAUM 3,020,439

HIGH EFFICIENCY TRAVELING WAVE TUBES Filed July so, 1958 5 sheets-sheet :s

. J. n l l y v /q V 0. 25o 9/4 92 l 4oov 8-0" 2L- 5400V o INVENTOR. E57. 7. ARIE L. EIEHENBAUM:

3,020,439 HIGH EFFICEENCY TRAVELING WAVE TUBES Arie L. Eichenbaurn, Princeton, NJ., assignor to Radio l Corporation of America, a corporation oi Delaware Filed .luly 30, 1953, Ser. No. 752,12 14 Claims. (Cl. S15- 3.5)

This invention relates to high efliciency traveling wave tubes and particularly to means for maintaining the average velocity of the electron beam thereof substantially constant throughout its traversal of the slow wave propagating structure.

In one form of a conventional traveling wave tube aV helical, slow wave-propagating structure is provided between an electron gun and an electron collector. An RF signal is fed to the helix at one end and an amplified output of the input signal is taken Vfrom the other end of the hel'm. The electron beam, which is focused to pass axially along and inside of the helix, is modulated by interaction with the RF Wave on the helix during its traversal of a first portion of the helix and then in turn gives up energy to effect an amplification of the wave.

' The phase velocity of an RF wave along a traveling wave tube helix of uniform diameter and pitch is constant. In order to obtain optimum operation so as to achieve the highest possible gain, it is necessary that the average velocity of the electron beam be maintained nearly equal to the phase velocity of the RF wave traveling along the helix. However, in giving up its own axial kinetic energy in order to produce amplication, the electron beam is slowed down. This slowing down of the electron beam increases exponentially along that portion of the helix Where amplification is eected. Eventually the beam velocity becomes so much below the phase velocity of the traveling wave that amplifying interaction between the Wave and the beam is impossible, and no further ampliiication of the Wave is produced. Such a condition places a limitation upon the amplification, or gain, of the tube. R. W. Peter proposed in U.S.` Patent No. 2,817,037, issued December 17, 1957, that the D.C. space potential along the electron beam of a traveling wave tube could be varied by providing a series of helices and connecting them to increasing D.C. potentials. By so doing energy can be supplied to the electron beam so as to maintain its average velocity constant.

An object of this invention is to provide a novel and improved arrangement for maintaining a constant average beam velocity in a traveling wave tu-be by providing one or more D.C. potential establishing electrode elements along and adjacent the helix so as to ing beam potential therealong. l

Brietly, according to my invention, a non-propagating D.C. potential establishing electrode means is disposed adjacent the helix or other slow wave propagating structure with a constant phase velocity in a traveling wave produce an increastube and may, for example, comprise a plurality of rings` or cylinders disposed around and along the helix and connected to successively increasing D.C. potentials. The accelerating axial electrostatic elds produced by the potentials on the rings penetrate to within the helix where they establish an increasing beam potential gradient therealong. Such increasing potential gradient serves to maintain the beam velocity constant notwithstanding the fact that the beam is giving up energy to the RF wave in order to produce amplification thereof. Alternatively, the D.C. potential establishing electrode means may comprise a single ring in the form of a hollow frusto-conical member surrounding and extending along at least the nal portion of the helix with its smaller diameter end adjacent the electron collector end of the helix. A single D.C. po-

tential, higher than the helix potential, connected to the ZSAS atented Feb. 6, 1962 member serves to provide an increasing beam potential gradient within and along the helix.

In the drawings: 1

' FIG. l is an axial section view of a helix type traveling wave tubev and circuit Vembodying one form of my inventiongfand FIGS. 2, 3, 4, 5, 6, and 7 are partly schematic axial section views of other embodiments of my invention.

FiG. 1 shows a helix type traveling wave tube 10 coml prising a dielectric envelope 12 containing an electron gun 14 at one end and an electron collector 16 at the other. A slow Wave propagating helical conductor or helix 13 of uniform diameter and pitch is disposed between the electron gun 14 and the electron collector 16 and is axially aligned therewith. The helix 18 is supported on three dielectric rods 20, two of which are shown in FIG. l, `and a series of dielectric rings 22 which are notched to receive the rods 2t). One end of the helix 18 is provided with an axial extension 24 to couple with an RF signal supplied by an input wave guide 26. The other end of the helix 18 is similarly provided with an extension 28 to couple an amplied wave produced on the helix to an output wave guide structure 30.

According to my invention, a plurality of conductive rings 32 are disposed around and uniformly spaced along the helix 18. The rings 32 are connected to a voltage divider 34 which in turn is connected to two taps on a D.C. voltage source 35. Predetermined, desired voltages are also supplied by the voltage source 36 to the electron gun 14, the helix 18, and the electron collector 16. Means,

, such as an elongated electromagnet surroundinglthe envelope, (not shown) is provided for producing an axial magnetic field B as illustrated by the arrow.

The electron gun 14 is conventional and comprisesv a cathode 38, heater 39, focusing or shield electrode 4i), tirst anode 4'2 and second anode 43. In operation of the tube 10 according to my invention, electrons are emitted by the cathode 38 and are focused and accelerated by the shield electrode `40 and anode 42, respectively, into a beam of electrons which travel along the tube axially through the helix 13 Where they are collected by the collector 16. According to the principle of operation of the conventional traveling Wave tube, an RF input signal is fed through the input Wave guide 26 and coupled tothe extension 24 of the helix 18 from whence it is propagated along the helix at a constant phase velocity substantially less than thel velocity of light and coupled from the extension 28 at the other end to the output wave guide 3th. The initial beam velocity is adjusted, by varying the D.C. potential of the helix, to a value lsubstantially equal to the phase velocity of the traveling vWave on the helix. The RF signal coupled to the helix serves to velocity modulate the beam` during its traversal of thetirst portion of thehelix. During its traversal of the remainder of the helix the velocity modulated beam gives up ener-gy to the traveling RF wave to produce an amplification ofthe wave. To prevent a decrease of average axial beam velocity due to the energy given up thereby to the traveling wave, progressively increasing D.C. potentials are connected to the rings 32 to. produce an accelerating axial electric field. Since in the absence of the rings 32 -as provided by my invention the beam velocity would fall oif in an exponential manner,

the D.C. potentials connected to the rings 32 are preferably.

trated in FIG. l so as to give the desired exponential .increase to the beam potentials along the helix. In the The 1200 voltage drop across the` the rings 32.

I i According to rny invention the D.C. potentials applied f to the `rings 32 provide .electrostatic fields thcreabout.

arrangement as illustrated in FIG. l, the helix potential .of 2400 voltsmay be connected to the collector 16, 2000 volts to the anode 42, and or ground to the cathode and shield 3S and 40.

It should be noted that the D.C. potential ran-ge applied to the series of rings 32 in the illustrative embodiment of FIG. l is given as 1800-3000 volts only by way of example. Actually Athe potentials applied to the rings 32 may extendover a wider or narrower range or may be vhigher or lower relative to the helix potential depending upon other tube parameters. In the illustrative embodiment of FIG. 1 the helix potentiallies intermediate the extremes of the potentials applied to the rings 32. This condition serves to minimize potential difference between any one ring 32 and the helix, and as such, minimizes the danger of arcing therebetween. It willbe appreciated that according vto myv invention the helix potential could be such as to fall outside the range ofipotentials applied to for example, be tapped to supply-2400 volts to the helix 18 and some range of potentims, eg., 180() and 300D volts respectively to the ends of the voltage divider 46. In order to obtain the desired increasing beam potential gradient along the helix y18, the rings 44 are made of different lengths andare disposed `so thatthe longer ring 44a is adjacent the electron gun andof the helix 18, and that successive rings in the direction toward the collector end of the helix are of successively shorter lengths. The

-In an actual tubevmade according to the teaching of FIG. 2, three rings 44 were provided having lengths of 3.5, 0.5, Land 0.25: inches. i yThe helix of the tube was connected to a D.C.fpotential of 800 volts and the three,

, vrings 44 to potentials of 700, 900, ,andr i100 volts respec- T hesefelectrostatic fields penetrate into the interior of theV I 'helix 18z whereithey inlluence the velocity of the velectron beam.

Inasmuch as thek helix 13 is interposed between the, rings 32 and electron beam, the held effect of the rings v 32 is only partially felt by the beam. v`The influence of f the fieldsof the rings 32 :upon thebeam maybe calculated according to known principles of eld penetration through a grid structure. In the embodiment as illustrated in i ,'FIG. 1 this penetration might be in the order ofi/5 of the voltage Qdifference between the potential on rings 32 and on the helix 18. For example, the ring 32a, which has a potential of 3000'volts applied thereto, might prce` vide ian effective beam yaccelerating potential inside the` helix 18 opposite vthe ring 32a of approximately 2520 volts, i.e., [2400+1/s (S900-240ML i In the embodiment illustrated in FIG. l, nine rings 32 are used. Theoretically, there is no'upper limit to the numberof rings which can be used. The more rings used Y the smoother the transition of the beam potential along tively. A substantial improvementzof synchronismbetween the average beam velocity andthe travelingwave v phase'velocity was noted. v

FIG. 3 schematically illustrates an `arrangement wherein a plurality of ringsv50-59`, 4all of the same, size, are disposedv aroundand along .a helix 18. The embodiment of 62. The alternate oddnumbered rings 51, 53, 55 `57, and

tential.

FIG. 3 differs from that of FIG. .l in that periodic elec@v trostatioocusingy of the electron vbeam is provided in combinationv vwith an increasing beam-influencing ypo- 56, and 58 are supplied with increasing D.C. potentials byvconnection to aseries of taps una first voltage divider 59 are also supplied withincreasing, ibut considerably/.lower, D.C. potentials by connection to a .series of taps on a second voltage divider 64. The first voltage divider 62 the-helix. However, experience has shown that the use of i from approximately three to six rings in practicing my invention is preferred ,for practical considerations.

Although the optimum arrangement is to provide an exponentially increasing D.C. potential along the helix as illustrated by FIG. l, it will be appreciated that such is not an essential requirement to the practice of my inven tion. The uniformly spaced rings 32 of the tube 10 could be connected to uniformly increasing D.C. potentials. However, since in the absence of the `rings 32 the average beam velocity would drop off exponentially, optimum correction involves application of an exponentially increasing D.C. potential along the helix. One way of providing such an exponentially increasingpotential is as illustrated in FIG. 1 wherein the potential bearing rings 32 are uniformly spaced with exponentially increasing potentials being connected thereto. Alternatively, an exponentially increasing potential along the helix can be obtained by providing exponentially positioned rings along the helix with uniformly increasing potentials connected thereto. When thelatter arrangement is used, successive rings should be of exponentially decreasing length according to theirexponential positioning in order to produce the smoothest possible transition of D.C. potential along the helix. FIG. 2 illustrates such an alternative.

FIGS. 2 through 7 illustrate other embodiments of the invention. In these figures, the envelope 12, input and output wave guides 26 and 30, heater 319, shield electrode 40 and rst anode 42 of FIG. 1 are not shown, to simplify the drawing.

In FIG. 2 there is schematically shown a series of rings 44 disposed around and along a helix 18. The rings 44 are connected to a series of taps on a voltage divider 46 so as to vsupply uniformly increasing potentials to `successive rings along the helix 18. A voltage source 48 may,

may, for example, be connected to a voltagesource 66: v vto 2000 and 2800 volt taps and the second voltage divider 64 to the voltage source 66 to 490 and 1200 volt taps.

The helix 18 may, for example, be connected to the 1260 v volt tap on the volta-ge source 66. Thus, the rings serve to provide an increasing beam-inuencing potential along the helix 18, and any two adjacent rings, i.e., 52 and 53, being at substantially different potentials, serve to provide periodic electrostatic focusing of the electron beam. In accordance with the previously discussed teaching of optimum correction, the potentials supplied to either series of alternate rings may ybe made of an exponentially in creasing nature.

FIG. 4 illustrates a modification of the arrangement of FIG. l, wherein a series of rings 70` are disposed around and along a helix 1S and are connected to exponentially increasing D.C. potentials, but are `elongated and lapped over each other. Each ring 70 ismade up of a first tubular section 72 of one diameter and a second tubular section 74 of a greater diameter. The diameters of the two tubular sections are so relatively dimensioned that the greater diameter section of one ring can be telescoped over the smaller diameter section of an adjacent ring. By so overlapping the rings 70, an RF shield is provided around the helix 18. Moreover, the rings 70 can be so dimensioned that in overlapping with an adjacent ring, open quarter wave lines can be approximated lby the space 76 between the rings. Thus, the series of overlapped rings appears as a continuous RF shield along the length of the helix 18 and as such prevents leakage of RF energy from the helix. Such overlapping also serves to provide a smooth transition of D.C. beam potential increase from ring to ring along the helix 18.

It will, of course, be appreciated that the teaching of uniformly increasing potentials applied to exponentially decreasing sized rings, as illustrated in FIG. 2, can be incorporated with the overlapping feature of FIG. 4. Likewise, the combined features of periodic electrostatic focus- The alternate: even numbered rings 5l), v52, 54,

invention.

sped-astiV Vprojected along and inside the helix 18 by means including an annular cathode l80. An increasing beam potential gradient along the helix 18 according to my invention is established by a resistive rod 82 extending coaxially within and yalong the helix 18. The resistive rod 82 is so sized that it permits the hollow electron beam to pass along the helix, surrounding but out of contact with the rod 82.

In order to provide the desired increasing beam potential gradient along the helix 1S, the extreme ends ofthe resistive rod 82 are connected to a pair of taps on a voltage source 84. For example, the helix may be connected to a 2500 volt tap with 2000 and 3000 volts, respectively, being connected to the ends of rods S2. In order to provide an exponentially increasing beam potential gradient,

the resistance of the rod S2 may be variable along itsv length. This may be provided either by tapering the rod or making the rod of uniform'diameter but of an increasing resistive material throughout its axial extent.

FIGS. -6 and 7 illustrate preferred embodiments of my invention which have an advantage over the embodiments of FIGS. 1-5 in that only one extra D C. voltage, and hence tube lead-in, vis necessitated for practicing the In the embodiment of FIGJ the electrode means for effecting a progressively increasing beam potential gradient along the helix comprises a'single member in the form of `a hollow frusto-conical member 90. The member 90 is disposed around vand along the helix 18 with its larger diameter end adjacent the electron, gun end of the helix. A single D.C. potential is applied to the member 90 by a D.C. voltage source 92, which also supplies -a somewhat lower potential to the helix 18.

The unipotential frusto-conical member 90 serves to provide an increasing beam potential gradient along and Within the helix 18 by virtue of its progressively closer spacing thereto along the helix 18. It can be shown mathematically that the effective potential existing along and within the helix as modified by the member 90 is a logarithmic function of the radial spacing of the member 90 from the helix 18. The greater the spacing, the less is the penetration of the potential of the member 90 to within the helix. Such logarithmic function, being itself of an exponential nature, vserves to provide the compensation of an exponentially increasing beam potential gradient required for optimum correction. The desired compensation can therefore be provided by use of a conically tapered member, as illustrated. VThis logarithmic function can be mathematically approximated by well known equations used to express amplification in a triode as determined by a particular grid configuration. Reference is made` to 7.7 of Vacuum Tubes by Karl R. Spangenberg for mathematicall expressions defining such eiective penetration.

FIG. 7 illustrates the novel vportion of an actual traveling wave tube, constructed according to the teaching of FIG. 6, incorporating a hollow frusto-conical section 92 surrounding the helix 18 and extending approximately only the last V the length thereof. The approximate 4/5 remaining length of the helix adjacent the electron gun end of the tube was surrounded with a straight hollow cylinder 94 joined to the large diameter end of the frustoconical section 92. It will be appreciated that such a composite cylindrical-conical structure, although not providing the ultimate in exponential compensation, will provide exponential compensation over the final portion of the helix where velocity fall-0E of the electron beam is most pronounced. Hence, use of a straight cylinder 94 for `the first length of the helix permits the use of a much smaller diameter vacuum envelope than is required by the frusto-conical member 90 of FIG. 6.

In the tube of FIG. 7, incorporating the composite 1` center of the helix.

cylindrical-conical structure, a helix 1-8 having a mean diameter of 0.250 inch was provided. The axial length of the frusto-conical section 92 of the composite structure was Aapproximately 2.2 inches and had an I D. at the small end thereof of 0.275 inch and an I.D. at the larger end thereof of 0.400 inch. The cylindrical portion 94 of the composite structure, connected to the l-arge end of the truste-conical portion 92, extended for a distance of approximately 8 inches toward the electron gun of the tube.

The LD. of the cylinder 94 portion was 0.400 inch, the same as that of the larger end of the frusto-conical portion 92. The tube was operated with a D.C. potential of 4800 volts on the helix 18, and 5400 volts on the composite cylindrical-conical structure 92--94. For all practical purposes the average velocity of the electron beam was measured to be effectively constant `and in synchronism of the phase velocityof the traveling wave throughout the helix of the tube.

Several advantages are realized from a practice of my invention. efficiency of the tube. Since, according to my invention, substantial synchronism of beamand phase velocity is maintained, greater amplification of the traveling RF wave is obtained. And, since overall tube efficiency is equal to RF power out divided by D.C. powerin, an increased efficiency follows.- l

A second advantage resides in the improvedlinearity of amplification at high power levels. This is apparent since amplification falls off as the -beam and phase velocities fall out of synchronism. And since extraction of energy, or power, from the beam causes a loss of synchronism, and since such energy extraction is greatest at high power levels, it follows that linearity ofy amplification must necessarily suffer with high power amplification in the absence of compensation for the slowing down of the beam.

A third advantage results from auxiliary beam focusing obtained by virtue of the helix being immersed in an electro-static field more positive than that provided by the helix potential itself. Due to this immersing field supplied: by the adjacent electrode means according to my invention, an electron Within the helix exists in a eld more positive than the potential of the helix. Hence, a force is exerted on the electron tending to urge it'tow-ard the In event that an electron does have suicient transverse velocity to enable it to travel toward the helix plane, it. will immediately be subjected to the more positive field of the surrounding rings and be attracted thereto on through and away from the helix. Such a condition serves to greatly minimize helix interception of electrons.

It will also be appreciated that in accordance with the embodiment of FIG. 3, my invention can provide periodic electro-static focusing without the necessity of additional structure. Hence, maintenance of constant average beam elocity and periodic focusing' can be conveniently comined.

An additional, and very important, advantage of the preferred embodiment of my invention illustrated in FIG. 6 is the fact that only a single extra D.C. voltage, and hence tube lead-in, is necessitated in order to provide corrective beam velocity'compensation. Such, of course, is made possible by virtue of the use of the single potential-bearing frusto-conical member 90.

What is Yclaimed is:

1. A traveling wave tube comprising an electron gun and an electron collector defining an electron beam path therebetween, a slow wave propagating structure disposed along and continuously coupled to said path, said structure comprising means for propagating waves therealong at a constant phase velocity, and means comprising at least one electrode disposed in spaced relation to and along at least the final portion of said structure for establishing an accelerating electrostatic field in the direction of electron travel.

Foremost of these is the increased overall 2. A traveling wave tube according `to claim 1, wherein said electrode is `located on -t-he opposite side of said wave propagating structure from said vbeam path.

3. A traveling wave tube comprising an electron gun and an electron collector dening an electron beam path therebetween, a slow wave propagating structure disposed along and continuously coupled to said path, said structure comprising means for propagating `waves therealong at a constant `phase velocity, and means comprising at least one electrode disposed in spaced relation to and along at least the final portion of said structure for establishing an accelerating electrostatic iie'ld in the direction of electron travel, said means comprising an electrode spaced at decreasing distances transversely from said structure along at least said inal portion thereof in the direction of electron travel, and means applying a D.C. potential to said electrode somewhat higher than the potential applied to said wave propagating structure.

4. The traveling wave tube according to claim 1, wherein said electron gun is yannular to provide van electron beam along a hollow beam path and wherein said means comprises a resistive rod coaxially disposed relative to and within said hollow beam path, and means connected to the ends of said rod `for establishing an increasing voltage gradient therealong in Vsaid direction.

5. A traveling wave tube ,according to claim 4, wherein said resistive rod has a variable resistance per unit length increasing in the direction of electron travel.

6. A traveling wave tube comprising an electron gun and an electron collector delining `an electron beam path therebetween, a helix of uniform diameter and pitch coaxially disposed relative to and surrounding said elec tron ybeam path, and means comprising at least one electrode disposed in spaced relation to and along atleast the final portion of said helix for establishing a progressively increasing accelerating electrostatic field in the direction of electron travel Aalong said portion.

7. A traveling wave tube comprising an electron gun and an electron collector defining an electron beam path therebetween, a helix of uniform diameter and pitch coaxially disposed relative to and surrounding said electron beam path, and means comprising at least one electrode disposed in spaced relation to and along at least the final portion of said helix `for establishing a progressively increasing accelerating electrostatic iield in the direction of electron travel along said portion, said means comprising an elongated hollow `member coaxially surrounding said helix and having an `inside Vwall convergently tapered along at least said final portion of said helix in the direction toward said electron collector, and means applying a D.C. potential to said member somewhat Vhigher than the potential applied `to said wave propagating structure.

8. A traveling wave tube according to claim 7, wherein said hollow member comprises a hollow cylindrical portion of given inner diameter surrounding about the rst fourfths ofthe length of said helix, and a hollow frustoconical portion surrounding the Aremainder of `said helix with a larger diameter end of said frusto-conical portion joined to said cylindrical portion.

9. A traveling wave tube comprising an electron gun and an electron `collector deiining an electron beam path therebetween, 'a helix of uniform diameter and pitch dis posed along and `continuously coupled to .said electron beam path, and means including a plurality of rings dis- -posed around and along said helix in spaced relation thereto for establishing a progressively increasing accelerating electro-static field in the ldirection of electron -travel along said helix.

10. The traveling wave tube according to claim 9 wherein alternate .ones of said rings are connected to predetermined D.C. potentials higher than the D.C. poten tials connected Ato the two rings adjacent any one of said alternate rings.

11. A traveling wave tube according to claim 9 wherein Vthe midpoints of said rings are progressively closer spaced to each other along said helix in the direction toward said electron collector.

12. The traveling wave tube according to claim 11 wherein said 'rings along saidhelix are of progressively shorter length corresponding to the progressively closer spacing of said rings and wherein each ring extends along said helix `to within proximity of its adjacent ring.

1.3. The .traveling wave tube according to `claim 9 wherein at least seme of said rings comprise hollow cylinders each having a first axial section of a given inside diameter and a second axial section of an outside diameter less 4than said given diameter, said rings being overlapped with a portion .of the first section of one ring surrounding a `portion of the second section of an adjacent ring.

14. The traveling wave tube according to claim 13 wherein the spacing between any two mutually overlapped rings is `of such size as to provide open quarter Wave structures resonant at a predetermined frequency.

References Cited inthe tile of this patent UNITED STATES PATENTS 2,232,050 Clavier et al Feb. 18, 194] 2,291,462 Gardiner July 28, 1942 2,458,167 Horsley Jan. 4, 1949 2,610,308 Touraton et al. Sept. 9, 1952 2,652,513 Hollenberg Sept. l5, 1953 2,653,271 W'oodyard Sept. 22, 1953 2,807,742 Kenmoku Sept. 24, 1957 2,825,841 Convert Mar. 4, 1958 2,843,776 Tien July 15, 1958 2,880,353 Warnecke et al. Mar. 31, 1959 2,888,596 Rudenberg May 26, 1959 FOREIGN `PirrENTs 660,660 Great Britain Nov. 7, 1951 @ca -ree.. r 

